Methods of dynamically switching return channel transmissions of time-division multiple-access (TDMA) communication systems between signaling burst transmissions and message transmissions

ABSTRACT

A method of communicating on at least one channel group of a communication system provides improved data transfer efficiencies over conventional systems by using discrete transfer rates. The channel group is includes at least one forward channel for channeling data transmitted from a central station to a plurality of terminals. At least one return channel is included for channeling transmissions by any of the plurality of terminals to the central station. Changes in the discrete data transfer rate are optionally accompanied by a change in a slot timing rate based on a frame transmission rate used over the forward channel. The method also allows the assignment of a predetermined number of return channel slots for a transmit frame for at least one combination of forward and return channels having prescribed forward and return data rates.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 08/724,116 filed Sep. 30, 1996, now U.S. Pat. No. 5,923,648, whichis incorporated herein by reference.

This application is related in its disclosure to the subject matterdisclosed in the following applications, all by S. Dutta, all assignedto the assignee of this application, and filed on even date herewith:

Ser. No. 09/231,089, entitled “Methods of Load Balancing and ControllingCongestion in a Combined Frequency Division and Time Division MultipleAccess Communication System Using Intelligent Login Procedures andMobile Terminal Move Commands”; and

Ser. No. 08/724,120, entitled “Methods of Communicating overTime-Division Multiple-Access (TDMA) Communication Systems with DistinctNon-Time-Critical and Time-Critical Network Management InformationTransmission Rates”.

BACKGROUND OF THE INVENTION

This invention relates to message store-and-forward communicationsystems and particularly to radio-frequency store-and-forwardcommunication systems which operate typically over a predetermined,limited number of available communication channels. Features of theinvention are found to have particular application in satellitecommunication systems operating in a “star topology” over a fixed numberof channels.

The invention herein is described as an improvement over a well knownprior art system which is known as the “Standard-C communicationsystem”, see THE STANDARD C COMMUNICATION SYSTEM, N. Teller et al.International Maritime Satellite Organisation, London, England,International Conference on Satellite Systems for Mobile Communicationsand Navigation, 4th, London, England, Oct. 17-19, 1988, Proceedings(A89-36576 15-32). London, Institution of Electrical Engineers, pp.43-46, (1988). A communication system such as the Standard-C systemoperates in a “star topology”, hence between a “hub” and a substantialnumber of “mobile terminals”. The Standard-C system utilizes schemesknown as Time-Division Multiple-Access (TDMA) and Frequency-DivisionMultiple-Access (FDMA) to accommodate a large number of intermittentusers (intermittently used mobile terminals) to share alimited-bandwidth, allocated, radio frequency band.

In FDMA, the allocated band is divided into a first number of narrowsub-bands, each sub-band constituting a time-continuous channel. Thefirst number of channels are to be shared among a second number ofusers, where the second number, the number of users, is typically muchlarger than the first number of channels. An access protocol referred toas trunking is used to accommodate the relatively larger number ofusers.

In TDMA, an entire allocated band forms a wideband communication channelwhich is allocated to different users at different times. Data packetsfrom a given user may be interspersed with those of another user duringtransmission over the communication channel.

Known communication systems, including the above-identified Standard-CCommunication System, use a combination of FDMA and TDMA, wheretime-division multiplexing is used in the frequency-division multiplexedsub-bands. In the Standard-C system, forward (from the hub to the mobileterminals) data traffic is carried in a time-division multiplexed (TDM)forward channel which is received by all mobile terminals of the system.Other, combined frequency/time-division multiplexed channels carryreturn data traffic from the mobile terminals to the hub.

Both the forward data traffic and return data traffic consist of twotraffic components of data. A first traffic component of data necessaryfor “call-setup” and “call-teardown” includes data referred to broadlyas system data and more specifically as network management data. Thefirst traffic component of data is called “signalling” or signallingpackets (of data). The term “signalling” is used herein throughout torefer to this first traffic component of data and to a specifictransmission mode in communication from the mobile terminals to the hub.A second traffic component of data bears user information, such asmessages or data reports. Messages are typically user information,having been composed by the user, while data reports are typicallytelemetry-type information packets that are transmitted periodically bythe mobile terminals. The transmissions of the second traffic componentof data to transfer user messages is also referred to herein as“messaging”. User information is in the forward direction communicatedover typically fixed links to the hub. The hub stores the userinformation and selectively formats it as the second traffic componentof data into the frames for transmission over the TDM forward channel.Systems, such as the Standard-C system are therefore also referred to as“store-and-forward” communication systems.

The Standard-C system communicates at a fixed data rate of 600 bits persecond (bps). In communication over the TDM, or forward channel, data ofboth components are formatted into frames and are transmitted as asequence of consecutive frames over the TDM channel. The frame length isestablished at 8.64 seconds, such that during a 24-hour period aninteger number of 10,000 frames are transmitted over the TDM channel.The information, message and signalling packets, are scrambled, ½-rateconvolutionally encoded, and interleaved on a frame by frame basis.Decoding of received frames of information is also done on a frame byframe basis. Considering that the transfer of messages or “messaging”also requires “signalling” in both the forward and the return direction,decoding delays become additive and result in typical message transportdelays of several minutes, such that a command-response type oftransaction cycle may take place over a time period of about fiveminutes.

Past applications of the Standard-C system in global communications havetraditionally involved communications between a fixed shore station (thehub) and any one of a number of mobile terminals which were ship-based.In such maritime environment, message transport delays of severalminutes were not considered to be unacceptable.

In contrast to a relative indifference to time delays in shore to shipcommunications, user messages between a central trucking dispatch depotand a fleet of operating trucks tend to be more time-sensitive. Forexample, interactive communication is required when a driver is indifficulty, e.g. by being lost or in misunderstanding with a customer.Urgent messages to alert drivers of additions or deletions in pick-up ordelivery schedules while the trucks are already enroute are more therule than the exception. Thus, if a Standard-C communication systemoperates between a “land earth station” (“LES”) as a hub and a number ofmobile terminal equipped, land-based vehicles, existing messagetransport delays of several minutes for typical command-response typetransactions become undesirable.

Increasing the data transmission rate over the standard 600 bps datarate of the Standard-C system would result in a higher message datacapacity per frame, thus allowing more mobile terminals to be servicedover the system. However a data rate increase over that of the prior art600 bps would not alleviate the system's inherent message transportdelays. On the other hand, relatively shorter frame lengths could speedup handshake operations between transmitting and receiving terminals andwould therefore reduce message transport delays. However, the use offrame lengths shorter than the standard 8.64 second frame length would,at any given transmission rate, tend to increase the ratio of networkmanagement data to user message data. Thus, shortening the frame lengthby simply increasing the data rate proportionally to the frame lengthreduction does not improve the aforementioned overhead ratio of networkmanagement information to user information. The frame structurecomprising both network management data and user message data wouldbecome compressed in time but remain proportionally the same.

Besides the undesirably long message transport delays which areexperienced in the described prior art communication systems, a problemof channel usage is experienced in such prior art systems. Communicationsystems, such as the Standard-C system, dedicates frequency-divisionmultiplexed return channels as either signalling channels or messagechannel. Thus, when the mobile terminals communicate with the hub, thefirst traffic component over the return channels, namely signalling, isassigned to the dedicated signalling return channels. Conversely, thesecond traffic component of user messages is assigned to the dedicatedmessaging return channels. It has become apparent that such usededication of available channels for communication from the mobileterminals to the hub results in an uneven utilization of the availablechannel capacities. As described above, signalling as used herein refersto the transport of protocol data packets for the purpose of networkmanagement, e.g. for call establishment and tear-down. Time divisionmultiplexing within a signalling channel is accomplished by means of theslotted Aloha protocol, a contention-based channel access algorithm thatdoes not guarantee packet delivery. In addition to the transport ofnetwork management protocol packets, it is also common to use thesignalling channel for the transport of short data reports, providedthey can tolerate non-guaranteed delivery, the use of signalling channelfor such purpose being referred to as “datagram” service. In contrast,user messages requiring guaranteed delivery are sent on separatefrequency multiplexed time-division multiple-access (TDMA) channelswhich are contention free. It is these latter type of TDMA channels thatare referred to as “message channels”. Access to the message channels iscontrolled by the hub, and is orchestrated through instructionscontained in protocol packets which are sent on the forward TDM channeland acknowledged or responded to over the return signalling channel.When typical user message communication takes place over the Standard-Csystem, it is common to reach a traffic congestion limit in the TDMforward channel and the signalling channels before reaching a congestionlimit in the message channels. Therefore, the message channels oftenremain still underutilized when the signalling capacity of a particularchannel group has become exhausted and further traffic has to be routedto a new channel group.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to optimize return channelcapacity in leased channel groups.

Yet another object of the invention is to adapt the capacity of a fixednumber of leased transmission channels to handle a range of ratios ofsignalling to messaging activities over return channels to more fullyutilize the transmission channels before switching to new channelgroups.

The invention is an improved time division multiple access (TDMA)communication system which operates to transmit data from a centralstation to a plurality of terminals over a forward channel of a channelgroup. Pursuant to the operation of the system, the data are formattedinto frames of a predetermined length or frame time period. The frametime periods are used, in turn, as timing periods for time-multiplexedtransmissions by the terminals to the central station on return channelsof the channel group. According to the improvement, periods of a lengthof at least one frame time period are selectively allocated on any ofthe return channels of the channel group to the transmission of messagedata from any designated one of the terminals in a continuous stream ofmessage data for the duration of any of the selectively allocatedperiods. Periods other than the periods selectively allocated to messagetransmission on any of the return channels are allocated to signallingtransmissions to occur within discretely defined signalling slotsoccurring within consecutive frame time periods.

Specific examples of preferred, preestablished frame time periods orframe lengths, without limiting the scope of the invention, are a framelength of 1.0 second for data rates of 600 or 1200 bits per second(bps), a frame length of 0.5 second for a data rate of 2400 bps, and aframe length of 0.25 second for a data rate of 4800 bps.

The formatted frames of data include time-critical network managementdata, such as return channel frequency assignments and slot timing forboth signalling and messaging modes of return channel communication. Thetime-critical network management data are consigned to a return channeldescriptor packet of each frame.

In a method of transmitting data over the forward channel in accordanceherewith, time-critical network management data are transmittedperiodically at a first rate, a frame transmission rate, to establish,for incremental periods of the length of a frame time period whether arespectively timed period is selectively allocated to the transmissionof message data in a messaging mode. Non-time-critical networkmanagement data are data that are not determinative of the transmissionmode of any such designated return channel during the respective periodas being the signalling mode or the messaging mode, and that may betransmitted periodically at a second rate which is less than the frametransmission rate.

Other features and advantages will become apparent from the detaileddescription set forth below.

BRIEF DESCRIPTION OF THE DRAWING

The detailed description of a preferred embodiment of the inventionshowing various distinctions over prior art Standard-C communicationsystems may be best understood when the detailed description is read inreference to the appended drawing in which:

FIG. 1 is a simplified schematic representation of a communicationsystem, showing in particular a satellite relayed time division multipleaccess message communication system which includes features of thepresent invention;

FIG. 2 is a simplified schematic representation of a Land Earth Stationof the communication system showing a satellite protocol processor (SPP)and channel units (CU) in greater detail and in conjunction with anetwork management subsystem (NMS);

FIG. 3 is a more detailed functional diagram of a TDM or forward channelmodulator of the channel units represented in FIG. 2;

FIG. 4 is a more detailed functional diagram of a representative one ofthe return channel demodulators for demodulating return channel signalsreceived by the land earth station, as schematically represented in FIG.2;

FIG. 5 is a schematic diagram of major functional blocks of arepresentative one of the mobile terminals in FIG. 1, showing particularfeatures pursuant to the present invention;

FIGS. 6a-6 h show a group of vertically arranged time bars drawn insubstantial temporal relationship to each other, the time barsrepresenting a frame pursuant to the prior art in FIG. 6a, and frames ofdifferent types and their characteristic arrangement pursuant tofeatures of the present invention in FIGS. 6b-6 h;

FIG. 7 is a schematic time bar diagram illustrating propagation delaysand delays due to data processing requirements as they occur in thecommunication system of FIG. 1, and their effects on multislotoperations;

FIG. 8 is a schematic diagram of the structure of return channelcommunications with alternate signalling and messaging periods pursuantto a feature of the invention;

FIG. 9 is a diagram showing the structure of bulletin board of systeminformation including system information such as return channelfrequency data pursuant to features of the invention;

FIG. 10 is a diagram of a return channel descriptor packet structure inaccordance with the invention; and

FIG. 11 is a schematic illustration of different classes of data beingtransmitted at different rates.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

Referring to FIG. 1, the invention is described with respect to asatellite-relay communication system which is designated generally bythe numeral 100. The communication system 100 provides, as the preferredembodiment of the invention, bidirectional, star topology datacommunication having at one end of the communications link an exemplaryuser station 110. The user station 110 is assumed to be stationary or“fixed”, such as a truck dispatch center 110 or fixed user station 110.However, the stationary character of the fixed user station 110 is notcritical to the invention. There may be one or more fixed user stationswhich become coupled to the communication system 100, as indicated byalternate fixed user stations 111 and 112, for example. The exemplaryfixed user station 110 communicates to a plurality of associatedterminals 120, which may be structurally identical, as indicated bytheir numerical designation, though the scope of the invention would notpreclude the use of a number of structurally dissimilar mobileterminals. Also, when a plurality of fixed user stations 110, 111, 112,etc., are incorporated in the communication system 100, each one of theseparate fixed user stations would, in a general scenario, seek tocommunicate with its own group or plurality of terminals 120. Forexample, the fixed user station 110 is described as communicating with afirst set of the terminals 120, which are in the preferred embodimentmobile terminals 120, while the fixed user station 111 communicates witha second set of the mobile terminals 120. As an example, the fixed userstation 110 may be envisioned to be a truck dispatch center 110, andeach of a plurality of mobile terminals 120 may be a truck-mountedmobile terminal 120, each associated with a truck, the routing of whichis controlled by the dispatch center 110.

User message communications via the communication system 100 iscontemplated to be bidirectional, such that any of the mobile terminals120 may originate communications with the corresponding fixed userstation 110, just as the fixed user station 110 may communicate with anyof the mobile terminals 120. Communication typically takes place viaconventional communication facilities which are referred to hereingenerically as “fixed links”. A fixed link 135, consequently, connectsthe fixed user station to a central station, such as a land earthstation (LES) 140 of the communication system 100. Similarly, additionalfixed user stations 111 and 112, etc., would be connected to the LES 140via their own respective fixed links 136, 137. The fixed links 135, 136and 137 of the preferred embodiment are contemplated to be eitherprivate, terrestrial, leased lines from a long-distancetelecommunication service provider, a terrestrial public packet datanetwork, or a VSAT fixed satellite network.

The LES 140 communicates with the mobile terminals 120 via a satellitedish antenna 150 and a relaying satellite repeater 155, transmitting andreceiving return transmissions over a plurality of distinct frequencymultiplexed RF communication channels referred to collectively as achannel group 160. Transmit frequencies and bandwidths of each of thechannels are assigned and controlled in the United States by the FederalCommunications Commission (FCC). Transmissions between the LES and thesatellite 155 take place on frequencies assigned in the Ku frequencyband, while transmissions between the mobile terminals 120 take place onfrequencies assigned in the L frequency band.

FIG. 1 further depicts a secondary receiving antenna 156 which receivesL-band transmissions for closed loop control of uplink transmit power,automatic frequency control and time synchronization for signaldemodulation at the LES 140. For example, the closed loop feedbacksignal received via the secondary antenna 156 conveniently provides asignal propagation delay correction which is necessary for burstdemodulation at the LES 140.

As in prior art Standard-C communication systems, each channel groupincludes a single TDM channel or forward channel 161. In thecommunication system 100, pursuant to the invention, reliable dataservice with automatic repeat requests (ARQ) is provided over both of anillustratively representative number of two (2) return channels 162 and163. It is to be noted, however, that the communication system 100 mayoperate with only a single return channel, such as return channel 162,in contrast to prior art systems.

In contrast to the communication system 100, prior art Standard-Ccommunication systems operate with designated signalling and messagingreturn channels. A prior art signalling return channel may provide whatis referred to as “from-mobile datagram service” used for “unreliable”data reporting, hence, without guaranteed delivery. In essence, there isno confirmation by the LES 140 of a receipt of the data report. Reliable“from-mobile” data service, providing guaranteed delivery, could, on theother hand, only be provided over designated messaging return channels.A typical Standard-C communication system might designate a plurality oftwo signalling return channels, and two or three messaging returnchannels, depending on an expected user message volume from mobileterminals. In such example, a prior art channel group might consist ofsix channels, one forward channel and five return channels withspecifically assigned signalling or messaging functions.

Referring back to the preferred embodiment in contrast to such prior artStandard-C system, the return channels 162 and 163 of the present system100 do not have predesignated functions of either a signalling or amessaging channel. Instead, messages carried over each of the returnchannels 162, 163, according to the invention, are slotted in a mixedsequence of signalling and messaging transmissions. Thus, a single TDMor forward channel 161 operating with a single return channel 162, forexample, is sufficient to represent a basic, bidirectional channel group160 of a communication system 100, providing signalling functions andreliable data service with ARQ over such single return channel 162.Channel groups 160 can, consequently, be configured to include the oneforward channel 161 and an optimally selected number of generic returnchannels 162, 163, or more, based on message traffic projections over areturn channel link. Thus, for the channel group 160, the two genericreturn channels 162, 163 are representative of any number of returnchannels. An upper limit to the need for adding more return channels (inaddition to the return channels 162, 163) to a channel group is reachedwhen the capacity of the TDM or forward channel 161 of the particularchannel group 160 becomes exhausted. A properly selected number ofreturn channels in the channel group 160 will show a substantially equalutilization of all channels of the channel group 160.

FIG. 2 is a simplified schematic representation of elements whichexecute functions of the land earth station (LES) 140 of thecommunication system 100. A network management subsystem (NMS) 170 iscoupled via a control bus 172 to all functional apparatus groups of theLES 140 including an intermediate frequency (IF) transmit-receivesubsystem 174 and a radio frequency (RF) transmit-receive subsystem 176.Interaction with the IF and RF subsystems 174 and 176 oversees forexample system timing, uplink power and automatic frequency controloperations. The control by the network management system furtherincludes control over a return channel demodulator selection functionshown as switch control 177 in FIGS. 2 and 4.

In reference to FIG. 2, the control bus 172 further couples the networkmanagement subsystem 170 to the satellite protocol processor, theoperations of which include virtually all digital data processing priorto modulation in the forward channel direction and subsequent todemodulation in the reverse channel direction of information flow. Theoperations include assembling outgoing network management informationand received user messages in formatted blocks or frames of digitalinformation. User messages are received over fixed links 135, 136, 137from a respective fixed user station 110 (shown in FIG. 1). As shown inFIG. 2, any number of the fixed links, as are designated generally bythe fixed links 135, 136 and 137, terminate at a communication interface181 of the SPP 180. Operations of the communication interface 181 arecoupled to and are monitored and controlled by the network managementsubsystem 170 via the control bus 172, a termination of which is shownas being functionally coupled to the communication interface 181. Thecommunication interface 181 interfaces outwardly with the fixed userstation 110 as described. The communication interface 181 may alsocommunicate system busy signals or other network problem reports to thefixed user station 110. The communication interface is coupled forbidirectional data transfer to a message storage subsystem 182 whichtemporarily stores user messages. These stored user messages wouldeither have been received by the LES 140 from one of the mobileterminals 120 as referred to in the above description of FIG. 1, or theuser messages have been transferred into storage by the communicationinterface 181.

Referring to further details of the SPP 180 in FIG. 2, the messagestorage subsystem 182 is connected bidirectionally, and is therebycommunicatively coupled to a queue manager subsystem 183, which in turnis coupled to a frame formatter and deformatter subsystem 184, wherebyeach of the communication links between the respective functional units182, 183 and 184 are bidirectional to process messages being sent to,and coming from, the fixed links 135, 136 and 137. Each of thesubsystems 182, 183 and 184 are monitored and controlled by the networkmanagement subsystem 170 as schematically shown by respectiveterminations of the control bus 172.

The queue manager subsystem 183 retrieves user messages from the storagesubsystem 182 to most efficiently fill frames of data to be transmittedvia the TDM or forward channel 161, and stores in the storage subsystem182 decoded and properly addressed messages received by the LES 140 overone of the return channels 162 or 163. These latter stored messages areperiodically transferred by the communication interface 181 to arespective one of the fixed user stations 110, 111 or 112, based onaddresses associated with such messages. Optionally, the messages may beforwarded immediately to the respective fixed user station rather thanbeing stored for periodic retrieval.

The frame formatter and deformatter subsystem 184 assembles in thedirection of the forward channel the frames of information to betransmitted over the TDM or forward channel 161. The assembledinformation is provided by the subsystem 184 to a digital link 186. Theassembled information includes predetermined network managementinformation and any user messages which can be accommodated in availabledata byte positions in each such frame. In a return direction flow ofinformation, the frame formatter and deformatter subsystem 184 extractssuccessful burst demodulation information over a digital link 187 andconverts the user information into a format suitable for transfer to arespective one of the fixed user stations.

The digital links 186 and 187 functionally couple the frame formatterand deformatter subsystem 184 to a channel unit subsystem 190. Thechannel unit subsystem 190 provides a function of digital modulation inthe forward direction of data flow between the satellite protocolprocessor 180 and the analog IF and RF subsystems 174 and 176, and adigital demodulation function in the return direction of data flowbetween respective analog RF and IF subsystems 176 and 174, and thedigital SPP subsystem 180.

In the forward direction of data flow, a TDM modulator 191 receivesassembled frames of information over the digital link 186, modulates theframes of information, converts the modulated information to an analogsignal and transfers the data modulated analog signal to the IFsubsystem 174, as shown in FIG. 2.

FIG. 3 shows functions of the TDM modulator which are in general thoseof prior art Standard-C systems, differing in specifics and preferredoperations as noted. The known functions include scrambling of data in ascrambler 192. Scrambling enhances 0/1 transitions and is known tothereby enhance recognition or reception of transmitted signals. Thescrambled signal is convolutionally encoded for forward error correctionby a convolutional encoder 193. The encoder uses known coding methods,such as rate-½ convolutional code with prior art generator polynomialsbut applies such code in the preferred embodiment described herein todata transmitted at data rates of 2400 and 1200 as well as to thosetransmitted at a standard 600 bit per second data rate which is alsoused in the Standard-C communication system. At a data rate of 4800 bpsa punctured version of the rate-½ code is used to generate an effectivecode rate of ¾. A frame delimiter 194 adds a unique word to the encodeddata as a frame start sequence. The now fully assembled frame is passedto a digital modulator 195. Forward link modulation is preferably p/4gray-coded DQPSK with root raised cosine filtering to limit thebandwidth of the signal. The output of the digital modulator 195 is ananalog signal which is applied to the IF transmit-receive subsystem 174,as shown in FIG.2.

FIG. 4 shows a more detailed schematic representation of a demodulatorwhich is designated generally by the numeral 197. A feature of thedemodulator 197 is that it is a dual function demodulator whichfunctions either as a burst demodulator of received, discrete signallingpackets of information, or it functions as a continuous or messagedemodulator. Switching to one or the other operational modes iscontrolled by the network management subsystem 170. The demodulator 197may, however, include as a first separate functional element a burstdemodulator 198 which has the sole function of demodulating received,discrete signalling packets of information. A second separate functionalelement, a continuous or message demodulator 199 is activated, and theburst demodulator 198 becomes deactivated when the return channel, suchas return channel 162 (FIG. 1) operates in a messaging mode. Controlover activation of one or the other function is exercised by the networkmanagement subsystem 170 in the particular embodiment via the switchfunction 177. The switch function may be the operation of a hardwareswitch 177, or it may be a typical device select signal applied by theNMS 170. The switch 177 is representative of both. In both instances,the NMS controls via the switch 177 an operation of either the burstdemodulator 198 or the message demodulator 199. Also, as a furthermodification, the demodulator 197, instead of having integral therewitha burst demodulator 198 and a message demodulator 199, may feature theburst demodulator 198 as a physically separate unit from the messagedemodulator 199. Again, either one or the other of the distinctdemodulator units 198 or 199 would be activated by a switching operationover the switch 177 to demodulate either a burst of data representing ashort signalling information packet, or to demodulate a message modetransmission, which is typically longer than a signalling informationpacket, as received from one of the mobile terminals 120 shown in FIG.1.

FIG. 5 is a schematic representation of the mobile terminal 120, asshown in FIG. 1, but showing functional elements in greater detail. Themobile terminal 120 would differ from prior art mobile terminalsoperating with a prior art Standard-C communication system. In a priorart Standard-C system, received as well as transmitted data are all atthe same data rate of 600 bits per second. In distinction over the priorart, the mobile terminal 120 has, in the preferred embodiment, theability to identify the data rate of data received over a modulated RFsignal on a preset forward channel. The mobile terminal 120 of thepreferred embodiment has the further function of transmitting data at arate that is automatically set by a network management message receivedover the forward channel from the LES 140 (See FIG. 1).

The mobile terminal 120 depicted in FIG. 5 shows a signal path forreceived transmissions over the TDM or forward channel 161 as receivedby an antenna 121 of the mobile terminal 120. The antenna 121 eithertransmits or receives signals, and as such is coupled to atransmit-receive diplexing unit 122. The diplexing unit 122 switches theantenna 121 between transmitting and receiving modes. Received signalsare converted by an RF stage 123 and then by an IF stage 124. The IFstage 124 is coupled to an analog-to-digital (A/D) converter 125. Adownconverted analog signal output from the IF stage 124 is consequentlyapplied as an input to the A/D converter 125. A resulting digital signaloutput from the AND converter 125 is applied as a signal input to a DSPdemodulator 126. The DSP demodulator demodulates the input from the A/Dconverter 125. The DSP demodulation process ascertains the data rate ofthe received signals.

The demodulated digital signals are transferred as a digital data outputby the DSP demodulator 126 to a control processor 127. The controlprocessor 127 interprets network management instructions contained aspart of the received data output from the DSP demodulator 126. Thecontrol processor also transfers received user messages to a displayterminal 128 of typical data terminal equipment (DTE) 129. It will beunderstood by those skilled in the art that the display terminal 128 maybe any of a number of terminal devices which translate digital code intohuman-discernible format, as, for example, an LCD screen 128.

The data terminal equipment 129 further includes, pursuant to thedescribed preferred embodiment, a keyboard 130 as one of a number ofdevices for generating digital input messages into the communicationsystem 100 as described herein and referred to specifically with respectto FIG. 1. Other devices for entering digital data into the system maybe a laser operated code scanner, or a digital output of a globalpositioning sensor 131 as a device for generating position data whichmay be transmitted as routine data reports by the mobile terminal 120 tothe LES 140 (see FIG. 1) over one of the return channels 162, 163, thelatter also being shown in FIG. 1.

Based on received network management instructions, such as data transferrates, return channel frequencies and assigned time slots, the controlprocessor 127 of the mobile terminal 120 compiles user messages fortransmission at a designated one of a number of available data ratesover the designated return channel 162 or 163. Data identified assignalling data packets or user messages are transferred to a frequencysynthesizer 132 from which a data modulated local oscillator signal isapplied to the IF stage 124 and another unmodulated local oscillatorsignal is applied to the RF stage 123 to transmit the RF modulated dataduring the time slots reserved for the particular mobile terminal 120.

It is well a established practice to use frame periods of a sequence offrames of information transmitted over a forward channel of a TDMAcommunication system for determining the slot timing of the returnchannels. The communication system 100, as depicted in FIG. 1, also usesthis practice. However, because of a reduced frame length with respectto the frame length of prior art communication systems, a typicalcommand-response type of transaction over the described communicationsystem 100 may be completed in less than 10 percent of the time requiredby prior art systems.

Message transport delays include signal propagation delays oftransmissions relayed via satellite which may range between 239 and 277msec, approximately ¼ of a second. Propagation delays will not changepursuant to the present invention. However, a decoding delay caused by anecessity of receiving a single, complete prior art frame having alength of 8.64 seconds before decoding can start is reduced to about 11percent of the prior art delay in the preferred embodiment by areduction of the frame length to one second. However, reducing the framelength disadvantageously affects the ratio of necessary networkmanagement information overhead contained in each frame relative tototal data byte space available within the frame.

Referring to FIGS. 6a-6 h, there are shown schematic representations offrames in an approximate time relationship to each other. FIG. 6a showsa representative prior art TDM frame 210 which is 8.64 seconds inlength, containing 640 bytes, using the prior art data rate standard of600 bits per second. The frame 210 includes a bulletin board (BB) 211with a fixed number, 14, of data bytes which occupies 0.19 seconds ofthe frame length. The bulletin board 211 is followed by signallingchannel descriptor packets (SCDP) the number of which depends on thenumber of signalling channels (13 bytes per channel) in the channelgroup of the prior art system. Three such SCDP units 212 are depictedwhich use up a period of 0.53 seconds of the length of the prior artframe 210. Thus, 0.72 seconds of 8.64 seconds of the frame length areoccupied by a network management information. If the length of the frame210 were shortened while maintaining the same data rate, the networkmanagement information within the frame 210 would take up relatively agreater percentage of the frame length, leaving less available time forthe transmission of user data, for example. The data transmissionefficiency over the forward channel declines. But even if the data rateis increased to shorten the frame length, the overhead ratio remainsconstant and an increase of the mL data rate from 600 bps to 4800 bpsallows a reduction of the frame length from 8.64 to 1.08 second whilemaintaining the same overhead ratio. An increase in the data rate overany given prior rate is known to increase the power requirement fortransmitting the data at such higher rate, proportionally to theincrease. State of the art mobile satellite systems do not favor thetransmission of data at rates greater than 4800 bps in a 5-6 kHzbandwidth. Thus, any reduction in the frame length to less than onesecond at the rate of 4800 bps would decrease the effective userinformation throughput over the forward channel, owing to an increase inthe percentile frame overhead devoted to network management information.FIGS. 6b through 6 h maintain substantially the same horizontal timescale as that of FIG. 6a to permit a comparison with the prior art frameof the percentile overhead devoted to network management information inframes of a selected length and formatted pursuant to features of theinvention. Comparative language in the further description with respectto FIGS. 6b-6 h relate to the prior art depicted in FIG. 6a, unlessotherwise noted.

FIG. 6b is a schematic representation of a login frame 215 designatedherein as a type A or first type frame. The login frame 215 has a lengthof 1.0 second (s) as a preferred modification of the frame lengthpursuant to this invention. The login frame 215 is also depicted at adata rate of 600 bps, as is the prior art frame 210. The frame 215 showsnetwork management information packets 216, 217 at the leading end ofthe frame 215. Whereas the relatively short login frame 215 (toreemphasize, the comparison is to the frame 210) of 1.0 second wouldtend to decrease message transport delays, the login frame 215 has,without doubt, an increased percentile of overhead. However, therelatively short frame length of the login frame 215 in combination withother features of the invention keeps increases in percentile overheadto a minimum, even at the 600 bps rate. At data rates higher than 600bps, formatting of frames pursuant to the invention, actually results inincreased effective information transfer rates, hence in a decrease ofpercentile overhead, as it becomes apparent from the description belowin reference to FIGS. 6c-6 h.

The login frame 215 features a leading return channel descriptor packet(RCDP) 216 of a length of about 0.1 second or about 10 percent of theframe length, which RCDP 216 is followed by a “long bulletin board”(LBB) 217 of network management data. The remaining length of the loginframe 215 is a message portion 218 which may carry call-related protocolpackets as well as user messages directed to one or more of the mobileterminals 120.

For a better understanding of described features of the invention andtheir advantages, certain terms used herein, though they may be used byand understood by those skilled in the art of communication systems, areclarified. The login frame 215 should be understood to be a frame offormatted data. The login frame 215 provides an excellent example forexplaining the meaning of certain terms used herein. The term data maybe used interchangeably with the term information.

There are broadly three categories of data involved in data transferover the system 100. A first category consists of network managementinformation or network management data. Network management data flowfrom the LES 140 to the mobile terminals 120. Network management dataare formatted into the frames which are transmitted over the TDM forwardchannel to the mobile terminals 120. Network management data pertain tothe overall operation of the system and include information whichadvises the mobile terminals 120 of the status of the system, as neededby the mobile terminals 120 to initially log into the system. Networkmanagement data also advise of the status of in-use return channels forany type of return communication by any of the mobile terminals 120 tothe LES 140.

A second category of data are “protocol data” contained in what is knownas “protocol packets”. Protocol data generally provide handshakinginformation which is required by the mobile terminals 120 or the LES 140in conducting particular communication sessions, in distinction over thenetwork management data which are directed to the operation of thesystem as a whole. In order to set up or break down a call, the protocolpackets are used both in forward channel and in return channeltransmissions. The protocol packets include requests andacknowledgements by the mobile terminals 120, or logical channelassignments and announcements or verifications by the LES 140 anddirected toward the mobile terminals 120. In transmissions from the LES140 toward the mobile terminals 120 over the TDM forward channel,protocol packets may be interspersed with messages in any of the frames.Transmissions of protocol packets from the mobile terminals 120 to theLES 140 occur in discretely spaced signalling slots on the returnchannels. Data transmissions by the mobile terminals 120 to the LES 140when made in a signalling mode are restricted to occur only within suchallocated time slots known as signalling slots.

A third category of data are message data. Message data are primarilyuser-generated. The transfer of user-generated messages is the essenceand purpose for which the communication system 100 exists.User-generated messages flow in both directions, outbound in the forwarddirection from the LES 140 to the mobile terminals 120 and inbound fromthe mobile terminals 120 toward the LES 140. There is a subcategory ofmessage data which are contained in data reports. Data reports are shortmessages which are typically automatically generated and periodicallytransmitted by the mobile terminals 120 to the LES 140 in a signallingtransmission mode.

A significant distinction of the use of data by the communication system100 over prior art systems is that the network management data aretransmitted at different rates, based on being further classified intoclasses of data each having different degrees of time-criticality. Eachclass of data is then transmitted at Rs distinct rate commensurate withthe respective degree of time-criticality of the data. In the preferredembodiment, network management data are classified into two classes oftime-critical, or real time data, and non-time-critical, or non-realtime data. The real time data and non-real time data are transmitted atdifferent intervals, and the operation of the communication system 100becomes more efficient as a result thereof. The two classes of networkmanagement data are appropriately labeled, respectively, as real timenetwork control data or “R-type” network control data, and non-real timenetwork control data or “N-type” network control data.

R-type network control data, i.e., real time network control data, aretime-critical data in that they are required at a frame input rate toensure that the instructions which the real time network control datarepresent will influence the operation to be controlled, namely, returnchannel transmissions, at the required time, namely, at alreadyidentified signalling slot periods which are timed by the frame and atthe frame rate. R-type network control data allocate in this regard,pursuant to the invention, designated, subsequent return channel slotsto either messaging or signalling modes, and received R-type networkcontrol data include two slot markers which relate to identifiedrespective prior and subsequent time slots. The slot markers indicatewith respect to the identified prior time slots whether signallingchannel bursts of data were successfully demodulated in each of theidentified slots of prior transmissions by the mobile terminal 120 (FIG.1). The data terminal 120 receiving the R-type network control data onthe TDM or forward channel 161 will examine the data (the status of theslot markers) to determine whether or not to retransmit if therespective prior transmissions were either not received or were receivedin error, for example through collision with another transmission on thesame slot from another mobile terminal. The slot markers indicate withrespect to the identified subsequent time slots whether such identifiedsubsequent time slots are reserved or may be accessed by any mobileterminal 120 on a contention basis. R-type network control data aretransmitted at a frame rate are contained as a first data packet in theRCDP 216 of each frame.

N-type network control data, i.e., non-real time network control dataare non-time-critical in that such data typically change infrequentlyrelative to the time span of a communications session, giving thenon-timecritical data a quasi-static character. This does not imply,that N-type network control data are not significant to the operation ofthe system as a whole. Login information such as frequency listings ofin-use return channels must be acquired by any one of the mobileterminals 120 before it can even attempt to log into a designatedchannel group. But the operation of the communication system 100 as awhole is benefitted when existing message traffic of possibly more than1000 currently logged-in mobile terminals 120 is given priority over amobile terminal which first seeks to log into the system. An analogy maybe drawn to vehicular traffic on a high-speed traffic route with respectto which there is a requirement for any traffic seeking to enter theflow of traffic to yield to already existing traffic so as to sustainexisting flow.

The login frame 215 is, pursuant to the invention, the only type offrame which includes the R-type network control data and all of theN-type network control data of the network management data, in additionto having protocol packages interspersed with user messages in themessage portion 218. The relatively short frame length of the loginframe 215 relative to the 8.64 second frame length of the prior artframe shown in FIG. 6a, increases the percentile overhead of networkmanagement data with respect to space which can be allocated to usermessages. The term “effective information rate” refers to the rate atwhich message data and protocol data, but exclusive of networkmanagement data, are transmitted over the forward channel. The effectiveinformation rate, or effective data rate, is thus related to the ratioof network management data to total available data capacity of a frame.

When a mobile terminal 120 becomes logged into the communication system100 (FIG. 1), the respective control processor 127 (FIG. 5) of themobile terminal 120 stores the bulletin board system information andupdates it only when changes are incurred during subsequent receipts ofthe bulletin board 217. Frequencies of in-use return channels 162, 163(see FIG. 1) are listed as a serial table in the bulletin board 217.In-use return channel data remain essentially the same over long periodsof time. The frame numbers are consecutive and change linearly withtime. Thus, internal clocks (not separately shown) of the mobileterminals 120 track, and interpolate between, frame numbers transmittedperiodically by the bulletin board. Available data rate combinations forthe forward channel and the return channels are contained in a binarycoded lookup table, which is stored in each of the mobile terminals 120.The bulletin board contains a binary code for the currently assigneddata rates. Before login can occur, the bulletin board information mustbe acquired by the respective mobile terminal 120.

According to the preferred embodiment, there exists a short bulletinboard frame 220 or frame (B), a second type of frame, which is depictedin FIG. 6c. The short bulletin board frame 220 is shown for the samedata rate of 600 bps as the login frame 215 and is also 1.0 second inlength. The short bulletin board frame 220 also features thedistinguishing leading RCDP 216 of R-type network control data which isfollowed by a short bulletin board (SBB) 221. The short bulletin boardis a subset of the long bulletin board (LBB) 217 in that N-type networkcontrol data which make up the short bulletin board 221 are also foundin the long bulletin board 217. However, certain other information, suchas a randomizing interval code, a calendar date code and theaforementioned tables of in-use return channel frequencies are omittedin the short bulletin board 221. A remaining message portion 222 of theshort bulletin board frame 220 provides, consequently, more bytepositions for user messages and protocol packets than the messageportion 218 of the login frame 215 in FIG. 6b.

FIG. 6d shows a third type of frame (C), a message frame 225, which alsohas a length of 1.0 second, but does not contain any N-type networkcontrol data, neither the subset of the short bulletin board 221 nor thecomplete set as in the long bulletin board 217. The message frame 225also begins with the return channel descriptor packet (RCDP) 216, whichconstitutes the time-critical or R-type network control data. Theremainder of the frame is allocated to a message portion 226 dedicatedto user messages and protocol packets to be transmitted over the TDM orforward channel 161 (FIG. 1).

FIG. 6e is a schematic representation of a frame arrangement referred toas a superframe 230. The superframe 230 is a sequence of one first typeof frame (A) or login frame 215 and seven third type of frames (C) ormessage frames 225, thus extending over a period of 8 seconds in length.The first frame of the superframe 230 consequently begins with thetime-critical information, the R-type network control data of the RCDP216, followed by the N-type network control data of the long bulletinboard 217. Each subsequent frame of the superframe 230, is a messageframe 225 and contains only the R-type network control data of the RCDP216, allowing the remaining capacity of the frame for messages andprotocol packets, hence, for message data and protocol data. It has beenfound that a login delay of no longer than 16 seconds occurring on 50%of attempted logins is clearly acceptable, and that a reduction in framelength does not require an association of all N-type network controldata or network management information with each frame.

It is noteworthy that only 50% of a deployed number of the mobileterminals 120 will take 16 seconds to log in, the remaining 50%achieving login within 8 seconds, even when the long bulletin board 218is only transmitted in alternate superframes 230. It is however, clearfrom the structure of the superframe 230, as described with respect toFIG. 6e that time-critical or R-type network control data are includedin every frame. The R-type network control data are required insituations where the mobile terminal 120 must provide an immediateresponse. Hence, R-type network control data are transmitted at a firstrate which is the frame transmission rate, while N-type network controldata, as required for login of a mobile terminal 120 (FIG. 1) isbroadcast periodically at a second rate which is less than the frametransmission rate at which the R-type network control data aretransmitted.

FIG. 6e depicts in the first frame position following the last messageframe 225 of the superframe 230 the second type of frame (B), or shortbulletin board frame 220, including the short bulletin board 221 asdescribed in reference to FIG. 6c. The short bulletin board frame 220 isfollowed by seven message frames 225 (of which only the first messageframe 225 is shown), to form with the short bulletin board frame 220 analternate superframe 235.

In reference to FIG. 6e, the LES 140 (in FIG. 1) arranges, consequently,in periodic repetition, one login frame 215 or first type of frame (A)and seven message frames 225, the third type of frame (C), into asequential grouping of eight 1-second frames thereby forming in eachrepetition a superframe 230 of the predetermined length of eightseconds. At the same periodic repetition rate, the LES 140 arrangesalternately one short bulletin board frame 220, type (B), and sevenmessage frames 225 or third type of frame (C) into a second type ofsequential grouping of frames, forming the second type of superframe235, which also is 8 seconds in length. The LES then transmits thesuperframes 230 and 235 in alternate succession, each at one-half of thesuperframe rate. Because the short bulletin board 221 is a subset of thelong bulletin board 217, it should be apparent that successive pairs ofalternately recurring superframes 230 and 235 constitute a “hyperframe”whose hyperframe transmission rate is half that of the superframetransmission rate. A hyperframe always begins with a login frame 215,containing the long bulletin board 217. The superframe, in relation to ahyperframe, is a shell within the hyperframe, as any of the framesdescribed herein are shells within the respective superframe.

In the preferred embodiment, the first type of frame 215 containing thelong bulletin board 217 is transmitted as the first frame of all evennumbered superframes 230 of a continuous sequence of superframes 230 or235, and the second type of frame 220 containing the short bulletinboard 221 is transmitted as the first frame of all odd numberedsuperframes 235 of the continuous sequence of superframes 230 and 235.Thus, login information, contained as a set only in the long bulletinboard 217 (or login bulletin board 217) may be acquired by a mobileterminal at the earliest occurrence of a periodic transmission of logininformation which is transmitted during the transmission of a firstframe 215 of even numbered superframes 230 at a rate of ½ of thesuperframe transmission rate, thus once during each hyperframe of asuccession of superframes 230 and 235. Time-critical data aretransmitted at a frame transmission rate which is a multiple of thesuperframe transmission rate of the superframes 230 or 235.

The foregoing suggests that variations are possible as to how, and atwhat frame rates, frames with time-critical information are transmittedwithin the structure of superframes or hyperframes, which in turn aretransmitted at respectively lower transmission rates than the frametransmission rate. For example, one may refer to the describedembodiment according to which alternate superframes 235 contain in aleading short bulletin board frame 220 N-type network control data whichare a subset of the set of N-type network control data providingcomplete login information in the long bulletin board 217. As amodification thereof, it is conceivable to substitute for the shortbulletin board frame 220 a message frame 225. The substitution forms asuperframe which has a length of the hyperframe as described above. Insuch modified case, the login information is still transmitted at a rateless than the transmission rate of the frames which make up thesuperframes, of once every 16 frames, although the partial N-typenetwork control data contained in the short bulletin board frame 220will no longer be available every 8 frames. It is also possible withinthe scope of the invention to transmit the login frame 215 at thebeginning of every superframe of a length of eight seconds. In suchmodified superframe structure, the maximum acquisition time for logininformation by a mobile terminal would be shortened from 16 seconds to 8seconds. The latter modification, of course reduces, with respect to thepreferred embodiment, the effective information rate, owing to anincrease in the percentile overhead of network management data.

Other features of the preferred embodiment include the ability totransfer data over both TDM, or the forward channel 161, as well as overan assigned return channel 162 or 163 (FIG. 1) at one of a number ofdesignated data rates. FIG. 6e depicts the superframe structure with therespective bulletin boards 217 and 221 corresponding in scale to theprior art bulletin board 211, as well as the RCDP 216 for three returnchannels corresponding in scale to the scale of tie three prior artSCDPs 212, all being shown at the same data rate of 600 bps as thestandard data rate over both the forward and the return channels of theprior art frame as depicted in FIG. 6a. Other contemplated forwardchannel data rates in accordance-with the preferred embodiment are 1200,2400 and 4800 bps, as described in reference to FIGS. 6f-6 h below.

FIG. 6f shows a superframe 240 of a sequential arrangement of eightframes, a first frame being of the first type (A), a login frame 241,followed by seven message frames 242 which are of the third type offrames (C). The frames 241 and 242 in the superframe 240 are structured,respectively, like the login frame 215 and the subsequent message frames225 of the superframe 230,depicted in FIG. 6e, except that the data ratein the superframe 240 and each of the frames therein is a 1200 bps rate.The RCDPs 243 which delimit the beginning of each of the frames 241, 242in the superframe 240 contain the same number of information bits as theRCDPs of the respective frames 215 and 225 depicted in FIG. 6e. Also,the bulletin board information in a long bulletin board 244 of the loginframe 241 is essentially the same as, and is structured in informationbits identically to, the long bulletin board 217 depicted in FIG. 6e.The distinction of the frames 241 and 242 in FIG. 6f over thecorresponding frames 215 and 225 in FIG. 6e is the time compression ofdata bytes, providing for a greater byte capacity within the same framelength of one second as that of the frames shown in FIG. 6e. The timecompression at the doubled data rate reduces the forward channeloverhead occupancy in each frame with respect to the total of availabledata bytes per frame by 50 percent.

The superframe 240 which is shown in its entire length in FIG. 6f wouldby definition be an even numbered superframe in a continuous successionof superframes, the even numbered superframes having in the first frameposition a login frame, the login frame 241 of data at 1200 bps in thisexample. The even numbered superframe 240 is followed by an odd numberedsuperframe 245 which begins with a short bulletin board frame 246, aportion of which is shown as the beginning of the next, alternatesuperframe at the 1200 bps rate.

Restructuring the frame by removing network management information whichis not time critical, such as by removing N-type network control datafrom all but one frame in a superframe, as a basis for reducing datatransfer delays by short frame transmissions, is seen as a departurefrom prior art practices. The sequencing of 1-second frames in asuperframe structure of a leading frame including N-type network controldata and seven message frames 225 (in FIG. 6e) and 242 (in FIG. 6f)achieves a major improvement in message transfer delays at both the 600bps and 1200 bps data rates. A tradeoff for increased transmission powerat the higher 1200 bps data rate is seen to be two-fold. Doubling thedata rate doubles the data transfer capacity. But also, as describedabove, the ratio of management network information overhead to databytes per frame is cut in half. The latter improvement intrinsicallyraises the throughput efficiency of the communication system 100 (FIG.1). By doubling the data rate, the effective information rate, whichexcludes the information contained in return channel descriptor packets(RCDPs), bulletin boards and any flush byte, is more than doubled.

Information transmission efficiencies achieved by doubling the data ratefrom 600 bps to 1200 bps as described with respect to FIG. 6f aresubstantially retained at further data rate increases with acorresponding reduction in the frame length as shown in FIGS. 6g and 6h. Such further increased data rates maintain the ratio of overheadinformation to the total byte length of each of the three types offrames (the login-type or type A, the short-bulletin-board-type or typeB and the message-type or type C) as that achieved by the 1200 bps datarate described in reference to FIG. 6f.

FIG. 6g schematically illustrates a superframe 250 which is composed of16 frames of 0.5 second in length and contains information at a datarate of 2400 bps. The first frame of the superframe 250 is a login frame251, thus of the first type or type A. The first type of frame 251 atthe beginning of the superframe 251 again identifies the superframe asan odd numbered superframe in a continuous stream of superframes ofinformation transmitted over the TDM or forward channel 161 (FIG. 1).Comparing the login frame 241 in FIG. 6f with the login frame 251 inFIG. 6g, the frames both have the same number of bytes, and, given theirapplication to the same channel group, the respective RCDP frameportions are identical in the number of bytes, as are the respectivebulletin boards. The same holds true when the short bulletin board frame243 of FIG. 6f is compared to a short bulletin board frame 253 of FIG.6g, or when one of the message frames 242 in FIG. 6f is compared to oneof the data frames 252 in FIG. 6g. In FIG. 6g, an RCDP 254 is at thebeginning of each of the frames. A long bulletin board 255 in the loginframe 251 is shown as being time compressed proportionally to timecompression of the frame 251 relative to the login frame 241 in FIG. 6f,and the short bulletin board 256 of the short bulletin board frame 253is similarly proportionately reduced in length. In each comparison thetotal number of data bytes and the ratio of network managementinformation bytes to the total number of data bytes for each respectiveframe type are the same.

It should also be noted that even though the data bytes in the frames ofFIG. 6g have been time-compressed with respect to those in FIG. 6f, thelength of the superframe 250 has not changed with respect to the lengthsof the previously described superframes 240 and 230. Thus, in contrastto the superframes 240 and 230, the structure of the superframe 250 at2400 bps pursuant to the preferred embodiment described herein contains16 frames, each being 0.5 s long, to maintain the same superframe lengthof 8 seconds.

FIG. 6h illustrates schematically data transfer over the TDM forwardchannel at 4800 bps. A superframe 260 at the 4800 bps data rate haspreferably 32 frames, each frame being 0.25 second in length, to accountfor the superframe length of 8 seconds. The first frame of thesuperframe 260, as illustrated, is also a login frame 261, thus of thefirst type or type A. The login frame 261 is followed by 31 messageframes 262. Pursuant to the described, preferred embodiment, loginframes (type A) occupy first frame positions in alternate occurrences ofsuperframes. Therefore, a frame occupying the first frame positionfollowing the last frame position of the superframe 260 is a shortbulletin board frame 263. Each of the frames 261, 262 and 263 begin withan RCDP 264. In the login frame 261 the RCDP 264 is followed by a longbulletin board 265. In the short bulletin board frame 263, the RCDP 264is followed by a short bulletin board 266. The frames 261, 262 and 263,as well as the RCDP 264, the long bulletin board 265 and the shortbulletin board 266 have, respectively, the same number of bytes as thecorresponding frames 251, 252 and 253 and the RCDP 254, the longbulletin board 255 and the short bulletin board 256 of the superframesdescribed with respect to FIG. 6g. The difference in each frame at the4800 bps data,rate in FIG. 6h with respect to a corresponding frame atthe 2400 bps data rate in FIG. 6g is that the frames at the faster datarate are proportionally compressed in time. The first type of frame 261at the beginning of the superframe 260 again identifies the illustratedsuperframe as an even numbered superframe 260 in a continuous stream ofsuperframes of information transmitted over the TDM or forward channel161 (FIG. 1). The second type of frame 263 is correspondingly followedby a sequence of 31 message frames 262 and forms therewith an alternate,odd numbered superframe 267.

At each of the different data rates (1200 bps, 2400 bps and 4800 bps) asthey apply to the illustrated superframes of FIGS. 6f, 6 g and 6 h,respectively, the total number of data bytes and the ratio of networkmanagement information bytes to the total number of data bytes for eachrespective frame type are the same. The difference between therespective frames of FIGS. 6f, 6 g, and 6 h at the respectivelyincreased data rates is a time compression, in proportion to the datarate increase, of the data bytes in the frames of FIG. 6g and FIG. 6hrelative to the data bytes in the frames of FIG. 6f. Significant is thatfor all data rates which may be available in a range of predetermineddata rates the lengths of the respective superframes is at the samepredetermined length and does not change, when the data transmissionrate is changed. As described, the preferred length for the superframeis 8 seconds, with a variable number of frames contained in eachsuperframe depending on the assigned data rate of transmittedinformation. As the number of frames in a superframe increasesproportionally to the data rate increase from 8 frames to 16 and 32frames, the ratio of N-type network control data (N-type network controldata being contained only in a first frame of each superframe) to totaldata byte capacity decreases. Thus, as the frame length becomes shorterat respectively higher data rates, the effective information rate overthe TDM forward channel increases.

Referring back to the representative communication system depicted inFIG. 1, the return channels pursuant to the invention are notpredesignated as either signalling channels or message channels, aswould be the case in prior art Standard-C systems. Instead, the returnchannels 162, 163 are generic and provide for the transmission by themobile terminals 120 of either signalling data in signalling informationpackets or of user messages over any return channel (e.g., 162, 163) ina channel group 160. The return channels 162, 163 are “time divisionmultiple access” TDMA channels which will be accessed by multiple mobileterminals 120 transmitting from widely dispersed positions of thecovered area, and involving correspondingly different propagation delaysfrom different ones of the mobile terminals 120 to the LES 140 (FIG. 1).Consequently, a slot occupancy time as observed at the LES 140 for anyone of a plurality of logged in mobile terminals 120 is expected todiffer with respect to that of any other.

Differences in slot occupancy times may be unmeasurably small or may beof a measurable length. The existence of position based variations inslot occupancy times for different mobile terminals is well known fromoperations of prior art Standard-C communication systems. The time slotson return channels are consequently discrete units, separated fromadjacent time slots by “guard spaces” of a predetermined duration toguard against variations in slot occupancy times from differentlypositioned mobile terminals. It is common for TDMA systems to use theframe period on the forward channel to time the slots on the returnchannels. For example, Standard-C systems use, at 600 bps forward andreturn channel data rates, and with a basic return channel “transmitframe” timing period of 8.64 seconds as established by the TDM forwardchannel frame length, 28 time slots of 308.3 milliseconds. Standard-Csystems provide for mobile terminals to transmit data packets of 263.3seconds during each of the time slots. The remaining gaps in each of thetime slots guard against data corruption by overlapping transmissionsfrom different mobile terminals.

It is to be noted in regard to data transmissions referred to herein,that data are transmitted as convolutionally encoded channel symbols.This general observation applies to the prior art as well as to datatransmission pursuant to the invention described herein. Knowntechniques of encoding bytes of data bits at a given data rate areapplicable. It is well known in the art that pursuant to such encodingtechniques, data are transmitted as “coded bits” at a symbol rate (sps)which is higher than the designated information data rate in uncoded“information data” bits per second (bps). Advantages of the describedembodiment over the prior art are not lost when existing encodingschemes are applied over the length of the frames as described herein.

As will be appreciated from the following discussion, variables, whichdid not exist in the operation of prior art systems, are introduced whenthe communication system 100 is operated according to the invention. Thepreferred operation of the communication system 100 is not onlydistinguishably changed over the prior art by transmitting data over theTDM forward channel in the relatively shorter frames, but also by usingframes of different length, depending on which of a number of availabledata rates being is employed for transmissions over the TDM forwardchannel 161. Moreover, return channel timing of slots will be affectedby different combinations of selected forward channel data rates andselected return channel data rates. To better understand the followingdiscussion and description of return channel transmissions in accordanceherewith, reference may be made to FIGS. 7 and 8 of the drawing, and toTABLE 1, set forth below.

Though using discrete, guarded time slots for information transfer overa TDMA return channel by a number of mobile terminals 120, the returnslot timing and multi slot assignments are distinct with respect to theprior art. According to the features of the invention, a range ofdiscrete data transfer rates are provided for both the TDM or forwardchannel 161 as well as the return channels 162 and 163 of the channelgroup 160, as set forth in TABLE 1 below. As already described, a changein the data rate may be accompanied by a change in the slot timing rate,based on the frame transmission rate used over the TDM or forwardchannel 161.

The preferred embodiment provides for 8 combinations of forward andreturn channel frequencies, as set forth in TABLE 1 below. A code forone of the combinations of forward and return channel data ratestransmitted in the (short and long) bulletin boards sets the data rateat which a receiving data terminal 120 will transmit data over theassigned return channel 162 or 163. The data terminal 120 uses theforward data rate information (in form of the code) in the long andshort bulletin boards to read the return data rate in the selectedforward/return data rate combination. The mobile terminal 120 senses theforward data rate in the DSP demodulator 126 (FIG. 5), basing aninterpretation of the data rate on detected phase changes of the channelsymbols. It is noteworthy that the mobile terminal 120 cannot derive theforward data rate information from a bulletin board, because it musthave the forward data rate information before it can demodulate thereceived symbols and read the bulletin board.

For each of the combination of forward channel and return channel datarates, a particular number of return channel slots per “transmit frame”is assigned. A “transmit frame”, quite like transmit frames used in theprior art, though different in length, is a timing unit transmitted overthe TDM channel 161 (FIG. 1) by which return channel transmissions aresynchronized. According to the preferred embodiment, which contemplatesdifferent combinations of data rates for forward and return channelcommunications, each transmit frame timing unit contains an assignednumber of return channel slots during which data may be transmitted by adesignated one of the mobile terminals 120.

Exactly how many return channel slots are preferably assigned to atransmit frame has been found to require a consideration of data ratesand transmit frame lengths on the return channels. In addition, adesirability to make the preferred operation described herein compatiblewith presently used modulating and demodulating apparatus further madeit desirable to size the data capacity of return channel slots to acceptinformation packets which contain the same number of code symbols as theinformation packets of prior art systems, for example 316 (64 uniqueword symbols followed by 15 bytes, 252 symbols. The number of returnchannel slots shown for each of the combination of preferred data ratesin TABLE 1 reflect these considerations.

For data reports or other protocol packets, which in the prior arttraditionally have been transmitted over signalling channels and thelength of which exceeded single slot allocations, multislot allocationshave been made. The prior art Standard-C systems employ multislotallocations spaced by delays of 2 or 3 frames. It has been found thatthe relatively shorter frame length, as used by the communication system100 relative to the prior art, no longer provides with delays of 2 or 3frames adequate time for data processing and propagation delays inspacing multislot signalling messages. Hence, multislot allocations inthe communication system 100 are spaced by delays of 4 to 6 frames interms of the relatively shorter frames, as compared to the prior art.Multislot frame spacing, pursuant to the invention, as described withrespect to the preferred embodiment for each of the differentcombination of assigned data rates and frame lengths, is set forth inthe last column of the TABLE 1 below.

FIG. 7 is a schematic time bar diagram illustrating propagation delaysand delays caused by data processing requirements and their effects onmultislot operations of the communication system 100 (in furtherreference to FIG. 1). The diagram shows time progression (t) along ahorizontal axis, showing delays that need to be taken into considerationwhen determining minimum offsets in terms of frames for the reservationof slots in multislot transmissions of signalling data. When atransmission of a data packet by one of the mobile terminals 120 to theLES 140 in the signalling mode requires more than one slot, the samereturn channel slot in particular subsequently occurring transmit frameperiods will be reserved, as required, to complete the transmission;provided, of course, that a first, contention-based signallingtransmission is received. However, before the mobile terminal 120 cantransmit any second or subsequent data packets over such reserved returnchannel slots, the mobile terminal 120 must first obtain feedback fromthe LES 120 that the first packet was indeed received and that therespective slot is indeed reserved in the expected return channeltransmit frame. If this is not the case, retransmission of the firstpacket, on a contention basis, must be attempted. Because of propagationand processing delays, slot reservation must be spaced by an establishedminimum number of intervening transmit frame periods for the mobileterminal 120 to look for an acknowledgement of receipt and a reservationof the signalling slot.

The timing relationships shown schematically in FIG. 7 illustrate aspecific example of transmit-receive delays between the LES 140 and anyone of the mobile terminals 120, showing transmissions over the TDMforward channel 161 at a data rate of 4800 bps and over the returnchannel at a data rate of 2400 bps. Uppermost and lowermost time bars(LES TX) are identical in that both represent a transmission of asequence of frames (“1”,“2”, . . . ) beginning at a reference time (t1).It is to be noted that, at the data rate of 4800 bps, FIG. 7 shows on anenlarged scale a succession of frames as described in FIG. 6h. Thus, afirst transmit frame period (1) is shown as the login frame 261 havingthe RCDP 264 of R-type network control data, followed by the longbulletin board 265 of N-type network control data followed by bytes ofmessages and protocol packets. The second transmit period (2), andsubsequent, consecutively numbered transmit frame periods (“2”,“3”, . .. ) are occupied by message frames 262, which contain no N-type networkcontrol data, so that the RCDP 264 precedes bytes of messages andprotocol packets. Because of the enlarged time scale with respect toFIG. 6h, FIG. 7 shows only a beginning sequence of the 32 frames in thesuperframe 260, and, hence, shows only part of a superframe period.

The beginning of each (TDM) transmit frame period shows a heavy line 271which represents transmit frame timing as it is transmitted by the LES140 over the TDM forward channel 161. The return channel frame timing isestablished for the entire system by the frame timing over the forwardchannel, identified by the frame period “T1”. At a data rate of 4800bps, the frame period T1 is 0.25 second. A second time bar from the top(MT RX) shows, again, the sequence of frames (“1”,“2”, . . . ) which isoffset with respect to the TDM transmit frame timing by a delay period“T2”. The delay “T2” represents a propagation delay for transmissions ineither direction between the LES 140 and any specified one of the mobileterminals 120. Depending on the location of the respective mobileterminal 120 the length of the propagation delay may vary, the delay“T2” being expected to vary over a range of 239-277 milliseconds(0.239-0.277 seconds). The delay period T2 is therefore expected to besomewhat greater than one frame period T1 of 0.25 s, but substantiallyof the same length. Thus, after the period T2, the frame “1” ofinformation, as identified by numeral 272, is being received by therespective mobile terminal 120 at its antenna 121. Timing lines 273represent the delayed TDM transmit frame timing at the antenna of themobile terminal 120.

For the mobile terminal 120 to decode the received frames of informationan entire frame of information has to be received, since the informationwas interleaved by the LES on a per frame basis and needs to bedeinterleaved accordingly. A resulting decoding delay is shown in athird time bar (MT DECODE). The decoding delay has the length of oneframe period T1 from the time that information is received at the mobileterminal 120. MT DECODE shows with a proper shift in time, wheninformation transmitted as frame “1” by the LES 140, becomes availableto the mobile terminal 120 as decoded data or decoded information 275.The decoded information 275 is shown to be reduced in length as it isexpected that the processor time required for decoding will be less thanreal time. The mobile terminal 120 can transmit information in asubsequent frame timing period 277 pursuant to instructions received inframe “1” from the LES 140, as shown in time bar (MT TX). Each of thereturn channel transmit frame periods “1”, “2”, “3”, . . . in the timebar (MT TX) identifies signalling slots, the mode and reservation ofwhich would have been communicated by the LES 140 to the mobile terminal120 in correspondingly numbered frames “1”,“2”,“3”, . . . of the timebars (LES TX). The availability of the number of signalling slots ineach transmit frame period depends on the length of the transmit frameand on the data rate of transmissions over the return channels 162 and163. A more detailed discussion of the preferred number of slots perframe for each contemplated combination of data rates is found below.

Any signalling transmitted by the mobile terminal 120 during the period277 corresponding to the frame period “1”, is received by the LES 140after the above-described propagation delay period of “T2” and over aframe period T1, identified by the numeral 279 in time bar (LES RX).Considering a relatively faster data processing capability at the LES140 as compared to that of the mobile terminals 120, the earliestoccurring RCDP 264 within which receipt can be acknowledged is that ofthe transmit frame period “7” shown at 281 in the time bar (LES TX). Thedecoded information transmitted by the LES 140 by TDM transmit frame “7”(at 281) is available as decoded data at the mobile terminal 120 in thecorresponding decoded data packet “7” as identified in the time bar (MTDECODE) by numeral 282. The respective flags in the RCDP 264 thereofindicate whether the prior signalling transmission by the mobileterminal 120 has been received and reserve the same slot for asubsequent frame period 284 for transmission use by the mobile terminal120. This subsequent transmit frame period at 284, also identified asframe period “7” in the time bar (MT TX), is the sixth transmit frameperiod of the mobile terminal 120 relative to the first frame in amultislot-packet transmission. The frame period 284 is the earliestoccurring transmit frame period at the mobile terminal 120 following thefirst multislot frame for which the slot allocation for a secondmultislot frame transmission by the mobile terminal could have beenconfirmed, and for which multislot reservations could have been made,considering all processing times and propagation delays.

TABLE 1 below sets forth in the last column preferred multislot framenumbers for different combinations of assigned data rates over the TDMforward channel 161 and return channels 162 and 163 according to thepreferred embodiment.

TABLE 1 {PRIVATE} Data Forward Return Frame Slots Multi- Rate Data RateData Rate Period per slot Code (bps) (bps) (s) Frame Frame No 000 600600 1.0 3 4 001 600 1200 1.0 6 4 010 1200 600 1.0 3 4 011 1200 1200 1.06 4 100 2400 1200 0.5 3 5 101 2400 2400 0.5 5 5 110 4800 1200 0.25 1 6111 4800 2400 0.25 2 6

Referring to table 1 for a specific example, when a bulletin board of afirst type of frame specifies in designated bit positions of a statusbyte a three-bit data rate code of “101”, all receiving mobile terminals120 (see FIG. 1) are instructed that forward channel communication isset to be at a data rate of 2400 bps and all associated return channels162, 163 (see FIG. 1) are operated at the return channel data rate of2400 bps. Column 4 of the table may be read in reference to FIGS. 6g,which depicts a frame length of 0.5 second for a bit rate of 2400 bps onthe forward channel. The sixth column of TABLE 1 shows the correspondingframe delay of five slots between slots in multislot packettransmission, not counting the current slot.

The fifth column in TABLE 1 specifies the number of slots which areavailable within a time period of one frame. In general, it will becomeapparent that, given a designated maximum size of a return channelsignalling packet of a fixed number of symbols, the symbol length beingdata rate dependent, the available slots per frame depend on the lengthof the frame and the data rate at which the return channel signallingpacket is transmitted by the mobile terminal 120. There is, however, oneother consideration which enters into a determination of how many returnchannel slots are available per frame at any given return channel datarate, and that is a consideration that slot guard time requirements areindependent of frame lengths or data rates on the return channel. Thus,as the frame lengths become shorter, ever, though the return channeldata rate may be increased proportionally to the frame length reduction,the number of permissible slots in each frame may have to be reduced tomaintain the minimum guard time.

Column 5 of TABLE 1 is further explained in reference to FIG. 8 whichdepicts the structure of return channel communication at data rates setby the binary code of “101” in the first column of TABLE 1. The datarate for return channel communication is set by the binary code “101” at2400 bps, and the frame length for forward channel communication at 2400bps is 0.5 second, as shown in column 4 of TABLE 1. In column 5 of TABLE1, there are five signalling channel slots allocated per frame for therespective data rate combination. In FIG. 8, evenly spaced markers 310show timing signals at the mobile terminal 120 (FIG. 1) which occur atthe transmit frame rate of 1/0.5=2 seconds, which is identical to thereceived forward frame rate. As discussed with respect to FIG. 7, thetransmit frames at the mobile terminals 120 are synchronized to theforward TDM frames as received by the mobile terminals, with thedescribed propagation delay T2. In FIG. 8, the spacing between twoadjacent frame markers 310 designates a period 311 corresponding to thelength of one frame of 0.5 second. At a return channel data rate of 2400bps, there will be 1200 bits in each 0.5 second frame period, whichtranslates into 2400 coded bits, or symbols, with a rate-½ code, perframe period, resulting in the rate of 4800 symbols per second (sps).Dividing an established return channel signal packet size of 316 symbolsin length into the frame period of 2400 symbols at the given rate, andneglecting the necessity of having guard spaces 314 of adequate length,for purpose of this analysis only, then the frame period 311 of 0.5seconds might have accommodated 7 return channel signalling slots.However, a minimum safe length for the guard spaces 314 between twoadjacent signalling slots 316 of no less than 30 milliseconds,nominally, dictates that the number of return channel signalling slots316 may not vary linearly with the return data rate or with the framelength, but that it must vary with due consideration given to guardspaces 314 of adequate length between adjacent ones of the slots 316.The guard spaces 314 discretely space each of the signalling slots 316by a minimum time interval. The assign ed return channel signallingslots per frame as set forth in TABLE 1 reflect the need for the guardspaces 314. In FIG. 8, the guard spaces 314 are located on either sideof nominal slot boundary markers 317, spacing an optimal number of sevensignalling slots 316 substantially evenly within each of the 0.5 secondframe periods 311.

When reading the following characteristic return channel operations withrespect to FIG. 8, reference may also be made to FIG. 1 with respect toreferred-to system elements. FIG. 8 illustrates the generic character ofthe return channels 162 and 163 as signalling and message channels.Though the return channels 162 and 163 function in either messaging orsignalling modes, it is preferred that a default state of any returnchannel is that of a signalling channel.

The significance for reverting the return channels 162 and 163 to asignalling channel state is best understood when the character of thereturn channels as TDMA channels is kept in mind. The use of one of thereturn channels 162 or 163 for message transmittal removes that channelfrom access by other mobile terminals 120 for the duration of themessage. When one of the return channels 162 or 163 is used as a messagechannel, as depicted by a message block 320 in FIG. 8, the messagetransmission will be continuous over the period of the assigned numberof signalling slots, except for a leading guard space 321 preceding thefirst assigned slot 316 and for a trailing guard space 322 following thelast slot 316 assigned to the transfer of the message. The entire timespan, starting with the leading edge of the first occupied slot andending with the lagging edge of the last occupied slot, becomes asingle, continuous message block within which slot positions as suchhave lost their character. It should be noted that the LES 140 allocatesperiods of the return channels 162 or 163 to messaging activities forthe lengths of periods as requested by any of the logged in mobileterminals 120. The allocation is made by the LES 140 in the returnchannel descriptor packets, such as the RCDPs 216 in FIGS. 6b-6 e, whichappear at the beginning of each frame transmitted over the TDM orforward channel 161. The transitions from messaging to signalling orfrom signalling to messaging will correspondingly occur at one of thespaced markers 310 representing the frame timing signal. A messagingallocation of a return channel period to any particular one of themobile terminals 120 is therefore made for the duration of any number ofcomplete frame lengths, less the leading and trailing guard spaces 321and 322 of, respectively, the first frame and the last frame allocatedto the message.

Slot reservations in multislot signalling transmissions by the mobileterminals 120 reserve future return channel capacity to a signallingmode. Multislot signalling packet transmission, consequently, extendsthe signalling mode for the duration of the respective multislottransmission. Any other mobile terminal 120 which contends for andaccesses the signalling mode of the return channel with another firstpacket transmission, while the return channel continues to remain in thesignalling mode during the completion of the first mentioned multislottransmission, could then cause the signalling mode to be furthercontinued based on a recurring number of spaced contentions forsignalling slots of the return channel while the return channel remainsin the signalling mode. Spaced, recurring multislot reservations arenoted, as described above, to bring about an inherent inertia inreleasing a return channel from the signalling mode to messagingactivities. It is therefore preferred to cut off all multislotsignalling on a particular return channel 162 or 163 on allocation ofthe particular return channel to a messaging activity.

The described advantageous features of the communication system 100 arecontemplated to be operated over existing or new satellite links such asthe satellite 155 as being representative of present and futuretechnology. The use of such satellite links may involve the use ofcommercially available transmission equipment and to adapt the describedfeatures hereof to operate in conjunction with prior art equipment, suchas equipment which operates in conjunction with the referred-toStandard-C communication system.

The description in FIGS. 6b-6 h of TDM channel frame structures inregard to the reduction of message transport delays explained thedesignation of non-time-critical network management information and itsinclusion in bulletin boards. Prior art Standard-C systems have theability to interpret the bulletin board network management informationat frame rate delays of 8.64 seconds. It will be understood by thoseskilled in the art that decoding and control apparatus, such as the DSPdemodulator 126 and the control processor 127 of the mobile terminal120, as described above with respect to FIG. 5, routinely interpret flagsettings in predetermined data fields of data packets, to initiatecontrol functions in response to such data flag indication. It will beunderstood that the assignment of data fields within any given networkcontrol data block is largely a matter of choice and that variousmodifications can be made to any preferred arrangement of data in anydata block without departing from the scope of the invention.Accordingly, the detailed description of the preferred embodimentincludes diagrammatic representations in FIGS. 9 and 10 of preferredarrangements of network control data within a representative longbulletin board (LBB) 217 or selectively modified short bulletin board(SBB) 221, and within a representative return channel descriptor packet(RCDP) 216, respectively.

FIG. 9 depicts schematically a representative data structure 350 as itwill be found in the above-described bulletin board 217. The data bytefields depicted in the structure 350 of data include a combination ofgeneral descriptor data as may be found in prior art Standard-C systems,combined with data which are specific to, and constitute features of,the invention. The format of data fields in the data structure 350 isconsequently preferably compatible with commercial satellite links, suchas AMSC-1. In FIG. 9, columns from left to right identify bit positionsof data byte positions in the data structure In 350, starting with bitnumber 8 on the left to bit number 1 on the right. The first or leadingbyte field of the structure 350 is shown at the top of FIG. 9, whileerror detection check sum field 351 appears at the bottom of the blockof the data structure 350. The first three byte positions of the packetstructure 350 are data fields designating packet type, length andnetwork version descriptors 352, 353 and 354. This type of informationis repeated at the superframe rate and is therefore also contained inthe short bulletin board 221.

A frame number field 355 in bytes 4 and 5 allocates 14 binary bitpositions to a sequential frame count over a 24 hour period. The framenumber field 355 also occurs in both the login bulletin board 217 and inthe short bulletin board 221 (FIGS. 6b and 6 c). One of the significantfeatures of the invention requires the elimination of standard bulletinboard information which is not time-critical from message frames otherthan those containing the short bulletin board 221 and the loginbulletin board 217. The frame number field 355 is, pursuant to thepreferred embodiment, included in the short bulletin board (SBB) 221 aswell as in the login bulletin board 217, to provide, when updatedcontinuously in accordance with consecutive occurrences, a count ofsuperframes. As described in reference to FIG. 6e, the frame numberfield 355 in the short bulletin board 221 would show an odd-numberedcount identifying an odd numbered superframe 235, while the loginbulletin board 217 would show an even-numbered count identifying aneven-numbered superframe 230. As described above, it is left up to themobile terminals to determine the actual frame count of frames receivedover the TDM channel. Such determination would be based on a linear timeinterpolation from the most recent receipt of either a login frame or ashort bulletin board frame, the data rate on the TDM channel, which isrelated to the frame length and its rate, and the count received fromthe respective most recent bulletin board or short bulletin board at thesuperframe rate. The three bit data rate code for both the forward andreturn channels is given in a data rate field 358 of bits 3,2 and 1 ofthe 11th byte position of the structure 350.

Bridging the 5th and 6th bytes is a 5-bit field which represents anactive return channel count designator 359, followed by a 3-bit fieldrepresenting a channel group congestion factor (CGCF) 360. The CGCF is aquantified loading or congestion indicator for the respective channelgroup. The CGCF 360 categorizes a current transmission congestion of thechannel group involving both forward and return channels as falling intoone of eight loading or congestion categories.

A 14-bit field 362 in byte positions 7 and 8 provides a frequency codeof the absolute frequency of the TDM or forward channel. Other bitpositions of the structure 350, up to the already identified data ratefield 358, which are not specifically identified hereby, are left opento indicate either vacant or spare data fields, or to indicate that theymay contain system status data which may be used in future enhancementsof the present system.

In the structure 350, the data byte fields between, and exclusive of,the data rate field (D.R.) 358 and the check sum field 351 are specificto the structure 350 of the long bulletin board 217 and do not form partof the short bulletin board 221. The short bulletin m board 221consequently contains a subset of the long bulletin board or loginbulletin board 217, the subset starting with field 352 and ending withthe field 358, except for the check sum field 351 which is also includedin the short bulletin board. Thus, in the short bulletin board 221, thecheck sum field 351 follows the status and data rate fields 363 and 358.

The data field 365 in byte position 12 provides a current calendar datecode to the mobile terminals. It is readily seen that an 8-bit codeallocation is insufficient to transmit a complete calendar datedescription. However, since the calendar date is not critical datatransmission, the complete date code is transmitted as a set of threedistinct data subsets and spans a period of three consecutiveoccurrences of the login bulletin board 217 in a continuous stream oftransmitted frames. A first subset of data (0, 7 bits) occurring in thefield 365 encodes the year of the century (0-99) as a binary number. Thesecond data subset (1, 0, 6 bits) encodes the month of the year (1-12)as a binary number. The third data subset (1, 1, 6 bits) encodes in thefield 365 the day of the month as a binary number. The complete calendardate is therefore repeated over a period of 48 seconds. The daily datesubset changes, of course, at a rate of 24 hours. The first daily datesubset is transmitted in the login bulletin board 217 of superframenumber six in each 24-hour count and for the last time in superframenumber 10800 of such 24-hour count.

Following the calendar date field 365, the structure 350 contains, in2-byte sets, a sequence of return channel frequency fields 367, 368 . .. 369, the fields constituting a serial table of frequencies of in-usereturn channels (channel(1), channel(2) . . . channel(N)) of any onechannel group. A randomizing interval designator field 371 follows inbyte position (n+2) the return channel frequency fields. The randomizinginterval designator field 371 is followed by the check sum field 351 inbytes (n+3, n+4) at the end of the structure 350.

From the above it will be understood that the length of the structure350 as it applies to the login bulletin board 217 is variable, dependingon how many of the return channel frequency fields are required, inother words, depending on how many return channels are active or in usein the channel group. As the number of in-use return channels increases,the serial table of listed frequencies in the bulletin board 217 becomessimilarly longer. The communication system 100 in FIG. 1 shows for theforward or TDM channel 161 of the channel group 160 a total of only twoactive or in-use return channels 162 and 163, that is, “n” is equal to2. Thus, for the depicted system 100 in FIG. 1, login bulletin board 217has, pursuant to the described features, a length of 19 data bytes. The19 data bytes are the described fixed 13 data bytes (1 through 11 and 2check sum bytes) of the short bulletin board 221, to which are added thebytes of the randomizing interval designator field 371 plus two 2-bytereturn channel frequency fields 367 and 368, each of the latterdesignating the frequency of a respective one of the return channels 162and 163. In light of the foregoing description it should be readilyapparent to those skilled in the art that if, in the communicationsystem 100 of FIG. 1, one of the two return channels, for example thereturn channel 163, were to become deactivated, the packet length of thelogin bulletin board 217 would change from 19 byte positions to a lengthof 17 byte positions. Thus in reference to FIG. 1, the return channel162 would remain as a sole in-use return channel operating inconjunction with the TDM or forward channel 161. Consequently, thebulletin board packet 350 would include only one active return channelfield 367 to list the frequency code for the return channel 162.

The above illustrates that, as the number of in-use or active returnchannels of any particular channel group varies based on the messagevolume carried by the particular channel group 160, so will the lengthof the login bulletin board 217. In a currently preferred operationalmode of the communication system 100, network operators may add to ordelete from currently in-use return channels of the respective channelgroup. In contrast to the login bulletin board 217, the short bulletinboard 221 (FIG. 6c) which does not carry the return channel frequencydescriptor bytes maintains a constant length of 13 bytes independentlyof the number of return channels in the channel group, pursuant to thedescribed embodiment.

FIG. 10 depicts a data structure 380 representative of data bytepositions in the return channel descriptor packets (RCDP) 216, whichhave already been described with respect to FIGS. 6b-6 h. From the abovedescription it should be apparent, that the RCDP 216 describes thestatus of each of the return channel slots of the transmit frame of themobile terminal 120 (FIG. 1) corresponding to the receive framecontaining the particular RCDP, as described above in reference to FIG.7. A single RCDP describes the slot allocations of all in-use returnchannels of a particular channel group. As the number of in-use returnchannels of a channel group varies, based on traffic conditions in suchchannel group, so varies the length of the RCDP. Data in the single RCDPtake the place of separate signalling channel descriptor packets whichare added to each TDM frame for each active signalling channel of achannel group, according to prior art requirements.

In reference to FIG. 10, the data structure 380 provides for a packettype field 381 and a packet length field 382 in byte positions 1 and 2,respectively, followed by as many return channel slot marker fields asthere are active or in-use return channels in the respective channelgroup. FIG. 10 shows a series of first, second and an nth return channelmarker fields 383, 384 and 385, showing the first return channel markerfield 383 (RET CHAN MARK(1)) as providing return channel markerinformation for a first return channel (such as return channel 162 ofthe channel group 160 in FIG. 1). The second return channel marker field384 identifies individual bit information positions for a second returnchannel of a respective return channel group. It should be noted that inany particular RCDP structure, such as the RCDP structure 380, allreturn channel marker fields of such structure, such as the fields 383,384 through the nth, a final marker field 385 contain the same identicalnumber and arrangement of marker bit positions as any other returnchannel marker field in such RCDP structure. It is further significant,that the number and order of return marker fields in any RCDP structurecorrespond to the number and order of the return channel frequency codefields of the corresponding login bulletin board structure 350.

The return channel marker fields carry in the 1st bit position of theleading byte position a slot preemption bit 386 “A”, and in the 8th bitposition of the leading byte position a message mode reservation bit,known as MMR bit 387. The MMR bit 387 has a significant function as asignalling-messaging mode flag or switch for each respective, in-usereturn channel 1, 2, . . . n. The MMR bit 387 is identified in FIG. 10by “M1”, “M2”, . . . “Mn” for, respectively, the 1st, 2nd and nth in-usereturn channels. When, according to the preferred embodiment, the MMRbit 387 for any of the in-use return channels is set to “1”, therespective return channel will be in the messaging mode for the durationof the respective frame period. When the MMR bit 387 is, conversely, setto “0”, the respective in-use return channel will be in the signallingmode for the duration of the respective frame period. As described withrespect to TABLE 1 above, the specified combination of data ratestherein for (see also FIG. 1) the forward channel 161 and any of thein-use return channels will determine the number of signalling slotswhich are available during a transmit frame. By setting thesignalling-messaging flag (the MMR bit) 387 to “1” or “0” for anyselected frame or succession of frames, the respective return channel isselected for transmissions of messaging or signalling information. Thisis a significant feature of the present invention, in that a pair ofchannels of a forward channel and a return channel can comprise a fullyfunctional channel group, whereas in the prior art systems, such as aStandard-C system, at least one signalling return channel and onemessage return channel was required, in addition to one forward channel,to constitute a full-function channel group. Thus, in a Standard-Csystem, at least three channels are required to achieve full servicefunctionality, whereas according to the present invention, two channelsare sufficient for the same functionality.

In further reference to FIG. 10, in the first byte position of thereturn channel marker field, six bit positions may be allocated to slotmarker bits. The MMR bit 387 sets the return channel mode for the entireduration of a frame with which the respective RCDP is associated aseither a messaging or signalling mode. When a logical “0” occupies theMMR bit position, the entire frame will be in a signalling mode.Correspondingly, a logical “1” state in the MMR bit position establishesa messaging mode for the entire frame length as already described withrespect to FIG. 8.

The MMR bit 387 is followed by two slot marker bit positions for eachslot in the corresponding frame period of the respective return channel.The logical state of the first slot marker bit indicates whether theprevious slot of a particular multislot packet sequence was correctlyreceived by the LES 140 (FIG. 1). The second slot marker bit indicateswhether the particular slot is reserved. However, as described abovewith respect to TABLE 1 and FIG. 8, the number of signalling slots inany return channel frame period depends on the data rate combination ofthe forward and return channels. Consequently, the number of slot markerbit positions and consequently the field lengths of the return channelmarker fields 383, 384 and 385 are data rate dependent. For each of thedata rate combinations in bps of the forward channel data rate to thereturn channel data rate (600/600, 1200/600, 2400/1200) there are threeslots per frame and, therefore, six slot marker bits are required,restricting the slot marker field to one byte. Similarly, for theforward/return channel data rate combinations of 4800/1200 bps and4800/2400 bps, there are, respectively 1 and 2 slot positions per frame,requiring also only a single byte for the slot marker field, leavingsome bit positions unused.

Forward/return channel data rate combinations of 600/1200 bps and1200/1200 bps have six slot positions per frame and require twelve (2per slot) slot marker bits, which are arranged in the 7-bit through2-bit positions of a 2-byte long return channel marker field. Aforward/return data rate combination of 2400/2400 bps has five slots perframe and requires ten slot marker bits with preferably 6 slot markerbits for slots 1-3 taking up the available bit positions of the firstbyte of a two-byte return channel marker field, while the remaining fourslot marker bits preferably take up the 7-bit through 4-bit positions ofthe second byte of the two-byte return channel marker field. Referringto FIG. 10, the slot marker allocation shown in the return channelmarker field 384 is one corresponding to a forward/return channel datarate designation of 2400/2400 bps. With brief reference to FIG. 9, suchdata rate combination would have been specified for the respectivesuperframe by either the bulletin board or short bulletin board in afirst frame thereof by the 3-bit code in the data rate field 358. Forthe illustrated 2400/2400 bps combination, the 3-bit code designation is“101”, as shown in TABLE 1.

A 2-byte field 389 is a check sum field used in error detection routinesduring the processing of the information within the packet structure380.

Having described the distinguishing features of both the TDM forwardchannel operation and of the return channel operation with respect tothe preferred embodiment of communication system 100, certain changesand modifications within the scope and spirit of the invention arepossible. Advantages derived through classification of networkmanagement data according to time-criticality, or more generally,according to their significance in the time domain, and a transmissionof such data at different transmission rates consistent with suchclassification has been explained with respect to the describedembodiment. Three transmission rates of (a) R-type network control dataat the frame transmission rate, N-type network control data, other thana complete set of login data, at a superframe transmission rate, andN-type network control data comprising the complete set of logininformation (inclusive of a serial table of return channel frequencycodes 367, 368 . . . 369) at ½ of the superframe transmission rate (ahyperframe transmission rate) have been described above. It would beunderstood that changes can be made, for example, in each of thetransmission rates or even in the number of transmission rate classes.

Classifying data, such as network management data, into a plurality ofgroups of data according to different degrees of time-relatedsignificance, referred to herein as time-criticality, and transmittingsuch data at different rates according to the grouping is best describedin reference to FIG. 11. In FIG. 11, there is shown a representativesequence 400 of transmission repetition units or timing units of whicheleven such transmission repetition units or timing units are shown,which are sequentially labelled by the numerals 401 through 411. Thefirst transmission repetition unit or the first timing unit 401 ischosen to contain four distinct data packets, containing data of,respectively, four different degrees of time-criticality. The datapackets are labelled, respectively 412, 413, 414 and 415, according tothe degree of time-criticality of data contained therein. The first datapacket 412 contains data which have the highest degree oftime-criticality. The fourth data packet 415 contains data which have,relatively to the other three data packets, the lowest degree oftime-criticality. The second and third data packets 413 and 414 contain,respectively, data of a second and third order of time-criticality.

In accordance with the time-criticality assigned to the data in therespective data =. packets 412, 413, 414 and 415, different rates arecorrespondingly assigned for transmitting the data. The data packet 412is transmitted periodically in every timing unit 401, 402, 403 . . . .The data packet 413 is transmitted periodically in every second timingunit 401, 403, 405 . . . . The data packet 414 is correspondinglytransmitted periodically in every third timing unit 401, 404, 407 . . .. The data packet 415 is transmitted periodically in every fourth timingunit 401, 405, 409 . . . . Thus, the network management data aretransmitted periodically at a transmission rate which directly relatesto the degree of time-criticality of the respective network managementdata. Network management data of a higher degree of time-criticalityrelative to other network management data of a relatively lower degreeof time-criticality are transmitted relatively more frequently or inperiodic transmissions having, relatively such other network managementdata a shorter period of recurring transmissions.

It is readily seen that the general example described with respect toFIG. 11 is similar to the more specific example of transmissions ofR-type and N-type network control data over the TDM forward channel 161(FIG. 1) of the communication system 100. The specific example of theR-type and N-type network control data includes the described,particular classification as a further feature to distinguish over themore general example in FIG. 11. In the preferred operation over the TDMforward channel 161, the disclosed hierarchy of a frame and superframe,or even a frame, superframe and a hyperframe are seen as teaching, moregenerally, a definition of any number of levels of time-criticality ofinformation, such as network management information, and a correspondingnumber of transmission periods for each of the defined levels. From theforegoing it will be understood, changes and modifications may be madenot only in the number of classes into which data can be placed as abasis for transmission at different rates, but also in the relativerates at which the different classes of data are transmitted.

Other changes and modifications may be made to the described embodimentwithout departing from the scope and spirit of the invention. Theappended claims are intended to cover any and al! such changes andmodifications within the scope and spirit of the invention.

It is claimed:
 1. A method of communicating on at least one channelgroup of a communication system, the channel group having at least oneforward channel and at least one return channel, wherein a centralstation transmits data over said forward channel of the channel group toa plurality of terminals, and transmissions by any of the plurality ofterminals to the central station on any one of the at least one returnchannel of the channel group, the method comprising the step ofcommunicating over at least one of the at least one forward channel andthe at least one return channel of the at least one channel group usinga range of discrete data transfer rates, wherein a change in thediscrete data transfer rate is optionally accompanied by a change in aslot timing rate based on a frame transmission rate used over theforward channel.
 2. A method according to claim 1, wherein saidcommunicating step further comprises the step of communicating over atleast one of the at least one forward channel and the at least onereturn channel of the at least one channel group using a range ofdiscrete data transfer rates, wherein a change in the discrete datatransfer rate is optionally accompanied by a change in a slot timingrate based on a frame transmission rate used over the forward channel,wherein a change in the discrete data transfer rate is optionallyaccompanied by a change in a slot timing rate based on a frametransmission rate used over the forward channel, and wherein the slottiming rate of the return channel is selected such that the returnchannel slots have a data capacity sufficient to accept informationpackets containing code symbols.
 3. A method of communicating on atleast one channel group of a communication system, the channel grouphaving at least one forward channel and at least one return channel,wherein a central station transmits data over said forward channel ofthe channel group to a plurality of terminals at a forward data rate,and transmissions by any of the plurality of terminals to the centralstation on any one of the at least one return channel of the channelgroup at a return data rate, the method comprising the step of assigninga predetermined number of return channel slots for a transmit frame forat least one combination of the at least one forward channel and the atleast one return channel having the forward and return data rates, thetransmit frame used to synchronize return channel transmissions by theplurality of mobile terminals.
 4. A method according to claim 3, whereinsaid assigning step further comprises the step of assigning thepredetermined number of the return channel slots for the transmit framefor the at least one combination of the at least one forward channel andthe at least one return channel having the forward and return data rateswith different combinations of the forward and the return data rates. 5.A method according to claim 3, wherein said assigning step furthercomprises the step of assigning the predetermined number of the returnchannel slots for the transmit frame for the at least one combination ofthe at least one forward channel and the at least one return channelhaving the forward and return data rates, responsive to the forward andthe return data rates.
 6. A method according to claim 3, wherein saidassigning step further comprises the step of assigning the predeterminednumber of the return channel slots for the transmit frame for the atleast one combination of the at least one forward channel and the atleast one return channel having the forward and return data rates,wherein 8 combinations of forward and return channels frequencies areavailable.
 7. A method according to claim 3, wherein said assigning stepfurther comprises the step of assigning the predetermined number of thereturn channel slots for the transmit frame for the at least onecombination of the at least one forward channel and the at least onereturn channel having the forward and return data rates, based on thereturn channel data rate and a return channel transmit frame length. 8.A method according to claim 3, wherein said assigning step furthercomprises the step of assigning the predetermined number of the returnchannel slots for the transmit frame for the at least one combination ofthe at least one forward channel and the at least one return channelhaving the forward and return data rates, wherein each transmit framecontains an assigned number of return channel slots during which datamay be transmitted by a designated one of the terminals.